7,212 research outputs found
Efficient computation of surface fields excited on a dielectric-coated circular cylinder
Cataloged from PDF version of article.An efficient method to evaluate the surface fields
excited on an electrically large dielectric-coated circular cylinder
is presented. The efficiency of the method results from the circumferentially
propagating representation of the Greenâs function as
well as its efficient numerical evaluation along a steepest descent
path. The circumferentially propagating series representation of
the appropriate Greenâs function is obtained from its radially
propagating counterpart via Watsonâs transformation and then
the path of integration is deformed to the steepest descent path
on which the integrand decays most rapidly. Numerical results
are presented that indicate that the representations obtained here
are very efficient and valid even for arbitrary small separations
of the source and field points. This work is especially useful in the
moment-method analysis of conformal microstrip antennas where
the mutual coupling effects are important
Efficient analysis of input impedance and mutual coupling of microstrip antennas mounted on large coated cylinders
Cataloged from PDF version of article.An efficient and accurate hybrid method, based on
the combination of the method of moments (MoM) with a special
Greenâs function in the space domain is presented to analyze antennas
and array elements conformal to electrically large material
coated circular cylinders. The efficiency and accuracy of the
method depend strongly on the computation of the Greenâs function,
which is the kernel of the integral equation that is solved via
MoM for the unknown equivalent currents representing only the
antenna elements. Three types of space-domain Greenâs function
representations are used, each accurate and computationally efficient
in a given region of space. Consequently, a computationally
optimized analysis tool for conformal microstrip antennas is obtained.
Input impedance of various microstrip antennas and mutual
coupling between two identical antennas are calculated and
compared with published results to assess the accuracy of this hybrid
method
Influence of the photon - neutrino processes on magnetar cooling
The photon-neutrino processes ,
and are investigated
in the presence of a strongly magnetized and dense electron-positron plasma.
The amplitudes of the reactions and
are obtained. In the case of a cold degenerate
plasma contributions of the considering processes to neutrino emissivity are
calculated. It is shown that contribution of the process to neutrino emissivity is supressed in comparision with the
contributions of the processes and
. The constraint on the magnetic field strength in the
magnetar outer crust is obtained.Comment: 8 pages, LaTeX, 2 PS figures, based on the talk presented by D.A.
Rumyantsev at the XV International Seminar Quarks'2008, Sergiev Posad, Moscow
Region, May 23-29, 2008, to appear in the Proceeding
Evolutionary history and identification of conservation units in the giant otter, Pteronura brasiliensis.
The giant otter, Pteronura brasiliensis, occupies a range including the major drainage basins of South America, yet the degree of structure that exists within and among populations inhabiting these drainages is unknown. We sequenced portions of the mitochondrial DNA (mtDNA) cytochrome b (612 bp) and control region (383 bp) genes in order to determine patterns of genetic variation within the species. We found high levels of mtDNA haplotype diversity (h = 0.93 overall) and support for subdivision into four distinct groups of populations, representing important centers of genetic diversity and useful units for prioritizing conservation within the giant otter. We tested these results against the predictions of three hypotheses of Amazonian diversification (Pleistocene Refugia, Paleogeography, and Hydrogeology). While the phylogeographic pattern conformed to the predictions of the Refugia Hypothesis, molecular dating using a relaxed clock revealed the phylogroups diverged from one another between 1.69 and 0.84 Ma, ruling out the influence of Late Pleistocene glacial refugia. However, the role of Plio-Pleistocene climate change could not be rejected. While the molecular dating also makes the influence of geological arches according to the Paleogeography Hypothesis extremely unlikely, the recent Pliocene formation of the Fitzcarrald Arch and its effect of subsequently altering drainage pattern could not be rejected. The data presented here support the interactions of both climatic and hydrological changes resulting from geological activity in the Plio-Pleistocene, in shaping the phylogeographic structure of the giant otter
Density-dependent interactions and structure of charged colloidal dispersions in the weak screening regime
We determine the structure of charge-stabilized colloidal suspensions at low
ionic strength over an extended range of particle volume fractions using a
combination of light and small angle neutron scattering experiments. The
variation of the structure factor with concentration is analyzed within a
one-component model of a colloidal suspension. We show that the observed
structural behavior corresponds to a non-monotonic density dependence of the
colloid effective charge and the mean interparticle interaction energy. Our
findings are corroborated by similar observations from primitive model computer
simulations of salt-free colloidal suspensions.Comment: Revised version, accepted to Phys. Rev. Let
Abnormal diffusion of a single vortex in the two dimensional XY model
We study thermal diffusion dynamics of a single vortex in two dimensional XY
model. By numerical simulations we find an abnormal diffusion such that the
mobility decreases with time as . In addition we construct a one
dimensional diffusion-like equation to model the dynamics and confirm that it
conserves quantitative property of the abnormal diffusion. By analyzing the
reduced model, we find that the radius of the collectively moving region with
the vortex core grows as . This suggests that the
mobility of the vortex is described by dynamical correlation length as .Comment: 13 pages, 8 figure
Exactly solvable mixed-spin Ising-Heisenberg diamond chain with the biquadratic interactions and single-ion anisotropy
An exactly solvable variant of mixed spin-(1/2,1) Ising-Heisenberg diamond
chain is considered. Vertical spin-1 dimers are taken as quantum ones with
Heisenberg bilinear and biquadratic interactions and with single-ion
anisotropy, while all interactions between spin-1 and spin-1/2 residing on the
intermediate sites are taken in the Ising form. The detailed analysis of the
ground state phase diagram is presented. The phase diagrams have shown to
be rather rich, demonstrating large variety of ground states: saturated one,
three ferrimagnetic with magnetization equal to 3/5 and another four
ferrimagnetic ground states with magnetization equal to 1/5. There are also two
frustrated macroscopically degenerated ground states which could exist at zero
magnetic filed.
Solving the model exactly within classical transfer-matrix formalism we
obtain an exact expressions for all thermodynamic function of the system. The
thermodynamic properties of the model have been described exactly by exact
calculation of partition function within the direct classical transfer-matrix
formalism, the entries of transfer matrix, in their turn, contain the
information about quantum states of vertical spin-1 XXZ dimer (eigenvalues of
local hamiltonian for vertical link).Comment: 14 pages, 9 figure
Analysis of Finite Arrays of Axially Directed Printed Dipoles on Electrically Large Circular Cylinders
Cataloged from PDF version of article.Various arrays consisting of finite number of printed dipoles on electrically large dielectric coated circular cylinders are investigated using a hybrid method of moments/Green's function technique in the spatial domain. This is basically an "element by element" approach in which the mutual coupling between dipoles through space as well as surface waves is incorporated. The efficiency of the method comes from the computation of the Green's function, where three types of spatial domain Green's function representations are used interchangeably, based on their computational efficiency and regions where they remain accurate. Numerical results are presented in the form of array current distributions, active reflection coefficient and far-field pattern to indicate the efficiency and accuracy of the method. Furthermore, these results are compared with similar results obtained from finite arrays of printed dipoles on grounded planar dielectric slabs. It is shown that planar approximations, except for small separations, can not be used due to the mutual coupling between the array elements. Consequently, basic performance metrics of printed dipole arrays on coated cylinders show significant discrepancies when compared to their planar counterparts. © 2004 IEEE
Hamiltonian Frenet-Serret dynamics
The Hamiltonian formulation of the dynamics of a relativistic particle
described by a higher-derivative action that depends both on the first and the
second Frenet-Serret curvatures is considered from a geometrical perspective.
We demonstrate how reparametrization covariant dynamical variables and their
projections onto the Frenet-Serret frame can be exploited to provide not only a
significant simplification of but also novel insights into the canonical
analysis. The constraint algebra and the Hamiltonian equations of motion are
written down and a geometrical interpretation is provided for the canonical
variables.Comment: Latex file, 14 pages, no figures. Revised version to appear in Class.
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